Traditional (pre-digital) animation techniques involved one or more artists hand-drawing an ordered sequence of separate image frames which were rendered at a suitable frame rate to provide the illusion of movement. Early digital animation involved artists creating individual image frames in a digital environment to produce an ordered sequence of digital image frames. An advantage of these early digital animation processes is that elements of a digital image frame (such as a relatively static background scene, for example) could easily be copied onto other image frame(s), thereby speeding up the process of creating a sequence of digital image frames. Relatively recently, 2-dimensional and 3-dimensional computer-based graphic (CGI) simulations have been developed which use computer-based models to simulate the movement of objects as between digital image frames and to thereby provide data which can be used to animate moving objects within a sequence of digital image frames.
There is a desire to provide computer-based simulation and/or animation of musculoskeletal systems (and/or similar systems) comprising a rigid-body (e.g. skeletal) system and a system of deformable, but volume preserving solids (e.g. soft tissue, such as muscle and fat), which interacts with the rigid-body system. Such animation may simulate movement of the musculoskeletal system. Such simulations may be used to generate corresponding animations of the musculoskeletal systems and/or similar systems, which animations comprise ordered sequences of complete or partial digital image frames often referred to as Computer-Generated Imagery (CGI) video data, CGI animation data or CGI moving picture data. There is a general desire that such animations, when rendered by suitable computer-based graphics engines, provide musculoskeletal animation that appears realistic to viewers when rendered. There is a corresponding general desire to minimize the computation expense associated with performing such simulations and/or generating such animations.
One commercially available prior art musculoskeletal animation system is the Maya™ Muscle system provided by Autodesk Inc. As understood by the inventors, the Maya™ Muscle animation system is based on simplified physics equations to achieve dynamic muscle effects, as opposed to physics-based volumetric muscle simulation involving Finite Element Methods or Finite Volume Methods and does not include volume-preservation or close-contact constraints. As a consequence, animations generated using the Maya™ system can appear relatively un-realistic to viewers. Another prior art musculoskeletal animation system is described by Lee et al. in Comprehensive biomechanical modeling and simulation of the upper body. ACM Transactions on Graphics (TOG), 28(4):99, 2009. As understood by the inventors, the Lee et al. system used a physics-based approach for modelling the shapes of muscles, but the Lee et al. model uses a muscle activation model using line segments to represent muscles and, consequently, can output soft tissue shapes that do not appear realistic when rendered. Another prior art musculoskeletal animation system is described by Teran et al. in Finite volume methods for the simulation of skeletal muscle. In Proceedings of the 2003 ACM SIGGRAPH/Eurographics symposium on Computer animation, pages 68-74, Eurographics Association, 2003. As understood by the inventors, the Teran et al. system incorporates a constitutive model which incorporates muscle fiber directionality. When muscles of the Teran et al. system are activated, they can exhibit relatively large forces in the direction of muscle fibers. However, such a model cannot be used to incorporate knowledge or desire of the muscle shape after activation and can still output soft tissue shapes that do not appear realistic. As understood by the inventors, none of these prior art animation systems allow users to specify input muscle shapes to which the soft-tissue model conforms.
Musculoskeletal systems comprise both relatively rigid tissue (e.g. skeletal bones) and relatively soft tissue (e.g. muscles and fat). While the musculoskeletal soft tissue is deformable, muscle and fat tissue exhibit volume preservation (within a typically normal range of pressure, temperature etc.). A prior art system for modeling soft solids in an Eulerian representation has been disclosed by Levin et al. in Eulerian solid simulation with contact, ACM Transactions on Graphics (TOG), 30(4):36, 2011. The Levin et al. system simulates soft solids using an Eulerian representation which resists volume change by varying a Poisson ratio of the soft solid between 0 and 0.5. As the Poisson ratio approaches 0.5, the soft solid material becomes relatively more volume preserving. The Levin et al. system (and other numerical methods and systems for simulating soft solids based on the Poisson ratio) tend to suffer from the so-called “locking phenomenon” wherein the numerical techniques used in such simulations tend to break down.
There is a general desire for musculoskeletal simulation systems to avoid or mitigate interpenetration of the various objects in the system. Some prior art systems (such as the Lee et al. system discussed above) do not address this interpenetration issue and therefore can appear un-realistic when rendered. In some prior art systems (such as the Teran et al. system discussed above and in the system described by Blemker et al. in Fast 3D Muscle Simulations Using a New Quasistatic Invertible Finite-Element Algorithm, International Symposium on Computer Simulation in Biomechanics (2005)) use computationally expensive mesh-based collision detection algorithms to avoid interpenetration of objects within the musculoskeletal system. There is a general desire to avoid or mitigate interpenetration of the various objects in a musculoskeletal simulation system while avoiding or at least reducing the need for computationally expensive mesh-based collision detection algorithms.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.